As one of techniques to increase the packaging density of optical fibers which are mounted in an optical fiber cable, a small-diameter and high-density cable as described in Patent Document 1 and Non-Patent Document 1 has been proposed. In the small-diameter and high-density cable, unlike a cable structure such as a slot rod or a loose tube, a sheath is directly provided on a cable core, and therefore, local bending being applied to an optical fiber cannot be avoided. In general, light confined in a core of an optical fiber leaks out of the core due to bending, that is, a loss occurs. For this reason, in the small-diameter and high-density cable, it is necessary to mount an optical fiber having good bending resistance.
As means for reducing a bending loss of an optical fiber, (1) increasing the refractive index (a relative refractive index difference Δ with respect to a clad) of a core region, (2) providing a trench layer, (3) providing holes around a core, and the like can be adopted. An optical fiber having a reduced bending loss (a low bending loss optical fiber) can be mounted in the small-diameter and high-density cable.